I could say FE13's Lunatic+ Chapter 2 is the worst, but it's so ridiculous that I don't think it's even worth mentioning.
Aside from that, the bridge between Crimea and Daien; in BOTH of the Tellius games, though I'd say FE10's bridge chapter is worse; oh, pegasi and herons fly right over the pitfalls? Well, why not have a few ballistas to help you out!